Should I expect this to work? If not shouldn't the docs indicate that message_tags is read only? Would love any feedback on this! Thanks you.

Update: I'm starting to settle into believing this won't work because only Facebook (or a user manually using Facebook) can tag another User in a message. I think this might be implied by the table here: https://developers.facebook.com/docs/reference/api/#publishing because it doesn't list message_tags as an argument even though, for instance, it doesn't list privacy but accepts it as an argument.